Welding material for austenitic stainless steels

ABSTRACT

A welding material with a chemical composition so adjusted that it forms a deposited metal which will contain as component elements not more than 0.15% C, 15.0 - 30.0% Cr, 8.0 - 40.0% Ni, not more than 2.5% Mn, not more than 1.5% Si, not more than 3.0% Mo, not more than 4.0% Cu, not more than 0.045% P, not more than 0.030% S, and not more than 0.30% Nb, and will also contain not more than 5% ferrite, characterized in that a suitable amount of Ta is chosen so that the deposited metal will contain 0.40 - 3.0% Ta, and the mixture is added to at least either filler metal (including the core wire for shielded arc welding) or flux.

United States Patent [1 1 Nishio et al.

[ 1 Oct. 21, 1975 WELDING MATERIAL FOR AUSTENITIC STAINLESS STEELS [73] Assignee: Mitsubishi Jukogyo Kabushiki Kaisha, Tokyo, Japan 22 Filed: June 29,1973

21 Appl. No.: 374,966

[30] Foreign Application Priority Data [58] Field of Search 75/128 G, 128 A, 128 W, 75/125; 29/193, 196, 196.1; 219/145;

[56] References Cited UNITED STATES PATENTS 2,801,916 8/1957 Harris 75/128 G 2,823,114 5/1958 Eberlee 75/128 E 2,889,223 6/1959 Fink 75/128 G 3,337,331 8/1967 Ljungberg 75/128 G Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Arthur J. Steiner Attorney, Agent, or Firm-Toren, McGeady and Stanger [57] ABSTRACT A welding material with a chemical composition so adjusted that it forms a deposited metal which will contain as component elements not more than 0.15% C, 15.0 30.0% Cr, 8.0 40.0% Ni, not more than 2.5% Mn, not more than 1.5% Si, not more than 3.0% Mo, not more than 4.0% Cu, not more than 0.045% P, not more than 0.030% S, and not more than 0.30% Nb, and will also contain not more than 5% ferrite, characterized in that a suitable amount of Ta is chosen so that the deposited metal will contain 0.40 3.0% Ta, and the mixture is added to at least either filler metal (including the core wire for shielded arc welding) or flux.

3 Claims, No Drawings WELDING MATERIAL FOR AUSTENITIC STAINLESS STEELS This invention relates to a welding material for austenitic stainless steels. More particularly, the invention concerns a welding material which, when used in welding austenitic stainless steels, can produce deposited metal free from crack due to the heat of welding and improved in corrosion resistance and strength for use at high temperatures.

A recent trend is that the conditions under which austenitic stainless steels are placed in service are becoming severer than ever, particularly with respect to corrosion resistance and high-temperature strength. To keep up with this trend it is theoretically required to use a welding material which would form a deposited metal on welding that has an austenitic structure with not more than 5% ferrite and also contains a carbideforming element, such as, niobium, so that a deposit with improved corrosion resistance and hightemperature strength can result. Actually, however, the deposited metal with 5% or less ferrite and with Nb addition has the disadvantage of frequent cracking on welding due to localized segregation of low-meltingpoint impurities because of the presence of Nb. In order to avoid this welding crack, it has been often inevitable to use a welding material adjusted in its chemical composition to yield a deposited metal with more than 5 to ferrite and therefore with a reduced harmful effect of Nb. The attempt has proved disadvantageous, too, because, when the deposited metal containing over 5% ferrite is placed in service at elevated temperature or when the deposit is stress relieved by annealing after welding, the ferrite in the metal will undergo transformation to a sigma phase with the consequence that the resulting deposit has low corrosion resistance and high'temperature strength, and particularly inadequate creep rupture strength.

Deposited metal with not more than 5% ferrite, by contrast, may be in service for long at elevated temperature or may be stress relieved by annealing without any sacrifice of its corrosion resistance, hightemperature strength and other desirable properties. The limitation of the ferrite content is thus essential for the improvement of various properties of the metal deposited by welding on austenitic stainless steels.

With the foregoing in view, we have made extensive investigations in search of a welding material that can form a deposit whose ferrite content is limited to 5% or less and which is, nevertheless, free from cracking upon exposure to the heat of welding and exhibits excellent corrosion resistance and high-temperature strength. As a result, it has now been found that a welding material in which niobium is replaced by another carbideforming element, tantalum, which is just as beneficial in improving the high-temperature strength and corrosion resistance, will give a deposited metal free from cracking due to welding despite the ferrite content of not more than 5% and which has excellent resistance to corrosive attack as well as exceptional strength at high temperatures.

Briefly stated, the present invention resides in a welding material of a chemical composition so adjusted that the elementary contents of the deposited metal thereby formed are not more than 0.15% C, 15.0 30.0% Cr, 8.0 40.0% Ni, not more than 2.5% Mn, not more than 1.5% Si, not more than 3.0% Mo, not more than 4.0%

Cu, not more than 0.045% P, not more than 0.030% S, not more than 0.3% Nb, and not more than 5% ferrite, characterized in that Ta is added in such an amount that the resulting deposit may contain 0.4 3.0% Ta and that the material of this composition is added beforehand to at least either filler metal (including the core wire of a coated electrode) or flux.

Tantalum, which is added to at least either filler metal of flux in accordance with this invention, has such a close affinity for oxygen, nitrogen, and carbon, that it reacts with oxygen and nitrogen in the welding arc to give tantalum oxide and tantalum nitride. Because of their high melting points the reaction products (TaO, m.p. over 3000C, and TaN, m.p. 3100C) form crystal nuclei as the molten metal begins to solidify and make the deposited metal very fine in grain size. Consequently the deposit has microcrystalline grains and is completely protected against cracking which may otherwise result from welding.

The low crack sensitivity of the deposited metal with refined grains may well be explained as follows. Generally the welding crack of austenitic stainless steel is a hot crack that occurs at a temperature just short of the solidification point, and this is caused by the opening due to the contraction stress on solidification after welding of the low-melting-point impurities that have precipitated at the grain boundaries of the deposit. The addition of tantalum gives Ta compounds (TaO and TaN) having high melting points which in turn effect grain refining of the deposited metal and thereby increase the total volume of the grain boundaries and decrease the concentration of the low-melting-point foreign matter at the boundaries to such an extent that the possibility of welding crack is eliminated.

It is also noted that tantalum has a stronger affinity for carbon than chromium and iron do. Therefore, the metal melted by the heat of welding arc is in a half melted and high-temperature state in its course of so lidification produces a tantalum carbide and fixes the carbon. This reduces the amount of free carbon to an extremely small amount and suppresses formation of noxious carbides, such as, chromium carbide, during cooling or reheating over 600C. Accordingly the corrosion resistance is retained and not affected in service. Moreover, the tantalum carbide with a high melting point (3827C) forms crystal nuclei in much the same way as the oxide and nitride to refine the grains of the deposited metal and prevent the welding crack.

While unreacted tantalum is carried into deposited metal, the high melting point (2996C) of Ta makes it impossible to form any foreign matter with a low melting point, and eliminates the danger of welding crack. In addition, the tantalum thus brought into the deposit strengthens the austenite structure and the solid solution, and therefore the deposited metal itself. A further benefit is that the fine precipitates of tantalum and the like exhibit such low aggregation rates when heated at high temperatures that they contribute strongly in improved the high-temperature strength of the deposited metal.

As stated above, the addition of tantalum to a com position which forms deposited metal containing not more than 5% ferrite is beneficial in avoiding welding crack and improving the corrosion resistance and hightemperature strength of the metal. Since the ferrite content of the deposited metal is too small for the formation of the sigma phase during heating at high temperatures, a good deposit can be obtained without any sacrifice of the corrosion resistance or hightemperature strength due to precipitation of the sigma phase.

According to this invention, as described above, the amount of tantalum to be added is limited so that it accounts for 0.4 3.0% of the deposited metal. The lower limit of 0.4% is set for the reason now to be explained. The minimum amount of tantalum required for stabilizing carbon and producing tantalum carbide is C% X 15. Actually C% X 20 or more is required because tantalum reacts with nitrogen and oxygen and accordingly the amount of tantalum that effectively combines with carbon is decreased. Since the minimum value of carbon content is approximately 0.02%, it follows that the minimum value of tantalum should be 0.02% X 20 0.4%. The upper limit of 3.0% is set because a higher percentage of tantalum would produce a brittle intermetallic compound TaFe, which in turn would embrittle the deposited metal and increase the crack sensitivity of the deposit.

The amount of niobium in the deposited metal is restricted to 0.3% or less for the following reason. In the tantalum-containing deposited metal 0.3% or less niobium can have nothing to do with welding crack, but because Nb is difficult to separate from Ta the former gains entrance as an impurity element into the deposit, and the addition of more than 0.3% Nb would lead to formation of Nb-based low-melting-point foreign matter and cracking of the deposit owing to the heat of welding.

For the reasons above stated, the deposited metal formed in conformity with the invention contains tantalum preferably in the range between 0.4 and 3.0%. On the basis of this content, the ranges of other additivies must be fixed. Tantalum when added to at least either filler metal or flux as taught above is not totally carried into the deposited metal because it is oxidized and consumed during welding with partial transfer into the slag. These possible losses must be taken into account in determining the amount of tantalum to be added. Usually the overall loss of an effective addition element in the course of welding depends upon the type of carrier in which it is added (i.e., in either filler metal or flux) and the method of welding to be adopted. Once these conditions are set, there will be practically no variation in the loss thenceforth. Our laboratory researches have given the following values. In the case of the coated electrode for shield arc welding and the welding material for submerged arc welding the tantalum added to the tiller metal alone will be carried into and left in the deposited metal in a yield ranging from about 40 to 70%, whereas the tantalum added to the flux alone will give a yield of about to 50%. When the element is added to the filler metal for inert-gas shielded arc welding, the yield will be 60 90%. From these the following conclusions are reached:

1. In the case of a coated electrode for shielded are welding:

a. When to be added to the filler metal (core wire) alone Ta should range in amount from 0.5 to 7.5%.

b. When to be added to the flux (coating) alone Ta should range in amount from 0.8 to 30.0%.

c. When to be added to both the filler metal and flux Ta should be used in an amount suitably combining the values (a) and (b) above. With the addition of Ta in the amount specified for any of the occasions given above, a desirable deposited metal will be obtained.

Ta proportions similar to those specified above will enable the welding material for submerged arc welding to form deposits as desirable as in shield arc welding.

2. In the case of filler metal for inert-gas shielded arc welding:

Ta added in an amount of 0.4 5.0% to the tiller metal will make possible the formation of a desired deposit. The grounds on which the composition of deposited metal is specified as above and the composition of the welding rod is so chosen as to obtain the particular deposit in accordance with this invention will now be clarified in connection with examples thereof.

First, welding of niobiumcontaining base metal, hotrolled stainless steel plate 347(SVS347HP conform ing to 11$ G4304), will be explained as an example. The chemical composition of the filler metal for such purpose is usually adjusted so that the deposited metal will contain more than 5 to 15% ferrite to prevent welding crack. This offers a serious disadvantage of low corrosion resistance and high-temperature strength due to transformation of ferrite to the sigma phase upon heating or while in service at elevated temperature. In contrast with this, a practically ferrite-free, crackless deposited metal with good high-temperature strength and corrosion resistance will result from welding in accordance with the present invention, that is, by the use of a Ta-containing electrode and in such a manner that the resulting deposit will contain not more than 5% ferrite. By way of illustration the compositions of deposited metals formed by commercially available electrodes and by an electrode of the invention will be compared in Table l. The cracking rates and mechanical properties of the deposits as determined by the Methods of Type C Restraint Welding Crack Test (JIS Z3155) are given in Table 2.

As can be seen from Table 2, the electrode according to this invention gives a deposited metal having a lower crack sensitivity and better high-temperature strength and corrosion resistance than those of deposits formed by conventional electrodes of commerce. In adding tantalum to a filler metal, care must be used to avoid segregation. When adding to a flux, the element should be as fine in grain size as possible and should be uniformly distributed in the flux.

Table 1 Element Amt. of Electrode C Si Mn Cr Ni Nb Ta ferrite Of invention 0.040 0.54 1.62 17.75 10.82 0.10 1.12 0 Of commerce Table 2 scribed. A mixture of electrolytic iron, ferrochrome, ferronickel, ferromanganese, ferrosilicon, and tanta- Conosion mime, Creep lum in suitably adjusted proportions for the intended he g) at 650C for rupture use is melted in air or an evacuated atmosphere in a l hours. stren th Crack cwsuausstest (kglmgmz) high irequency melting furnace. The resulting alloy El ing g y ms! imercrysml- 650C steel is drawn through a die into a wire form, and finecrate (65 c HNO;) line X1000 trade Cm/momh Corrosion hrs shed to a predetermined diameter. The w re finished in this way is cut to a suitable length and is used as a f 21in 0-0042 Good 25 bare filler metal for inert-gas welding or as a coated electrode with a suitable covering for the intended use. f Several examples of welding experimentally conzTf 71 00093 Fair to poor 19 ducted with filler metal s thus obtained are compared (2) 3 (in 0.0121 Fair to poor 13 with those conducted with conventional filler metals in Tables 3 and 4. Coated electrodes according to the invention are manufactured by applying a coating suitably prepared in view of the type of filler metal or of Next, a typical method of manufacturing an electhe welding conditions to be encountered over the filler trode in accordance with this invention will be demetal and then drying the coating.

Table 3 Chemical composition Chemical compn. of

Filler metal of filler metal deposited metal C Si Mn Cr Ni Nb Ta C Si Mn Cr Ni Nb Ta 1 Conventional 0.045 0.54 1.60 18.51 10.55 0.72 0.031 0.52 1.58 17.92 10.52 0.52

tiller 2 WithTa 0.047 0.52 1.66 18.52 10.71 0.02 0.39 01032 0150 1.59 17.98 10162 0.01 0.35

addition less than herein claimed 3 WithTa 0.048 0.51 1.62 18.81 10.72 0.03 5.12 0.033 0.50 1.52 18.52 10.68 0.01 3.51

addition more than herein claimed 4 Fillermetal 0.042 0.53 1.59 18.71 10.77 0.05 0.82 0.030 0.51 1.52 18.71 10169 0.02 0.53

olinvention 5 0.045 0.52 1.62 18.52 10.75 0.12 6 0.032 0.50 1.53 10.63 0.08 2.91

Amt. Crack- Corrosion test Creep of ing after heating rupl ferrate 650C 100hrs. str. Filler metal rite Huey Cu- (kg/ test Strauss mm) (cm/ (inter- 650C month) crys. X1000 corrohrs. sion) 1 Conventional 0 80 0009 Good 19 filler metal 2 WithTa 0 2* 0.012 Fair- 19 addition poor less than herein claimed 3 With Ta 0 0.007 Good 21 addition more than herein claimed 4 Filler metal 0 2* 0.004 Good 22 ofinvention 5 0 2* 0.002 Good 23 6 0 2* 0.001 Good 22 in crater.

TABLE 4 Welding tests with different coated electrodes for shielded arc welding Chemical composition Type Chemical compnr of of electrode (71) of deposited metal Coated coat electrode C Si Mn Cr Ni Nb Ta C Si Mn Cr Ni Nb Ta 1 Conven- 0.032 0.62 1.68 18.91 10.59 1.21 A 0.041 0.61 1.65 18.65 10.57 0.81

tional electrode 2 With Ta 0.031 0.62 1.71 18.92 10.58 0.01 0.38 A 0.040 0.62 1.65 18.71 10.51 0.01 0.20

addition less than herein claimed (to core wire) 3 With Ta 0.032 0.64 1.71 18.95 10.57 0.02 0.01 B 0.039 0.61 1.67 18.75 10.56 0.01 0.10

addition less than herein claimed (to coating) 4 With Ta 0.032 0.63 1.65 18.92 10.68 0.02 8.21 A 0.041 0.61 1.62 18.72 10.59 0.01 3.50

addition more than herein claimed (to core wire) 5 Electrode 0.031 0.62 1.68 18.92 10.59 0.05 2.41 A 0.039 0.61 1.64 18.72 10.57 0.02 1.51

of invention (with Ta added to core wire) 6 Electrode 0.032 0.65 1170 18.96 10.62 0.02 0.02 C 0.041 0.63 1.65 18.75 10.59 0.01 2.50

of invention (with Ta added to coating) Amt. Crack- Corrosion test Creep of ing after heating rup. ferrate 650CX100hrs. str. Coated rite Huey Cu- (kg/ electrode test Strauss mm) (70) (cm/ (inter- 650C month) crys. X1000 corro' hrs.

sion) 1 Conven- 0 7.5 0.012 Good 18.5

tional electrode 2 With Ta 0 2* 0.011 Fair- 19.0

addition poor less than herein claimed (to core wire) 3 With Ta 0 2* 0.015 Poor 18.2

addition less than herein claimed (to coating) 4 With Ta 0 22 0.009 Good 210 addition more than herein claimed (to core wire) 5 Electrode O 2* 0.008 Good 22.0

of invention (with Ta added to core wire) 6 Electrode 0 2* 0.007 Good 225 of invention (with Ta added to coating) in crater.

Where only a small number of welding rods according to thepresent invention are required, the end can be attained by coating the necessary number of core wires of ordinary austenitic stainless steel free from any additional element with a coating composition so prepared as to contain tantalum in the proportion specified above.

A few examples of coating compositions with which we conducted welding experiments are given in Table 5 10 As has been described above, the welding material Table 5 Examples of coating compositions Metal Metal Metal Metal CaCO 'IiO CaF Mn Cr Ni Fe-Si Ta Remarks A 35 25 5 5 3 2 Conventional coating B 34.5 25 25 5 5 3 2 0.5 With insufficient Ta C 32 25 25 5 5 3 2 3.0 Coating of invention Notes:

I. "Fe-Si" contains 80% Si.

2. An aqueous solution of sodium silicate is used as a binder.

We kneaded these coatings separately with an aqueous solution of sodium silicate and applied the mixtures on filler metals (core wires) of the electrode compositions shown in Table 4, each measuring 4 mm in diameter and 250 mm in length, to form 2 mm-thick coating layers thereon. The coatings were dried at room temperature for 3 days and then heated at 250 300C for 6 hours. Coated electrodes were thus prepared. With these electrodes 6 mm-thick hot-rolled stainless steel plate (Grade SVS347I-IP conforming to JIS 04304) was welded by AC arc welding with a current of 140A. The deposited metals thereby obtained were subjected to elementary analysis and some welding tests. The results are given in Table 4. It will be appreciated from the table that the electrodes with compositions to which tantalum is added in amounts within the range specified herein produce deposited metals superior in anticracking, anticorrosive, and high-temperature properties than the one formed with a conventional electrode and that the compositions with Ta contents outside the specified range give deposits inferior in resistance to cracking and corrosive attack.

Table 3 shows the results of some inert-gas arc welding tests conducted with Ta-containing filler metals. In

according to this invention gives a deposited metal containing not more than 5% ferrite and having excellent resistance to crack and corrosive attack and great strength at elevated temperature.

In other experiments filler metals of the invention were employed in welding steel of the DIN 4505 grade (conforming to DIN 17007 of the German standards) for anti-sulfuric acid use. Filler metals of DIN 4507 (DIN 17007), which are usually used for this purpose do not contain ferrite but do contain niobium in amounts of more than C% X 10 and naturally have the disadvantage of frequent cracking on welding. The properties of the deposited metals formed by the tiller metals of the invention were compared with those by the conventional filler metals. Table 6 shows the results of elementary analyses of deposited metals formed by inert-gas shielded arc welding with Ta-containing filler metals of the invention and with ordinary Ta-free ones. Table 7 compiles the test results obtained by the Methods of Type C,R estraint Welding Crack Test (JIS Z3155) and by the Methods of Copper-Containing Sulfuric Acid-Copper Sulfate Inter-crystalline Corrosion Test (JIS) and compares the mechanical properties of the deposited metals thus formed.

Table 6 Analytical values of deposited metals formed by inert-gas shielded arc welding Filler metal C Si Mn Cr Ni Mo Cu Nb Ta Of commerce (1) 0.035 0.35 1.51 0.013 20.05 20.60 1.90 2.05 0.45 (2 0.055 0.42 1.35 0.015 20.13 20.12 2.55 2.03 0.72

Of invention (1) 0.052 0.50 1.55 0.014 20.14 19.95 1.56 2.12 0.25 2.50 2 0.038 0.42 1.42 0.022 20.15 19.25 2.33 2.35 0.15 1.21

Table 7 Properties of deposited metals Cracking lntercrystalline Tensile test test corrosion test Filler metal cracking Tensile Elongarate As Heat treated strength tion Welded (650CX2HrAC) (kg/mm) Of commerce (1) 45.5 No No 59.5 48.5 (2) 65.4 No No 60.0 47.3 (3) 3* No Yes 58.3 49.9 Of invention (1) 2* No No 6L2 45.5 (2) 2* No No 60.5 47.8 (3) 1* No No 61.3 48.3 2* No No 60.7 49.9

As is obvious from the foregoing tables, inert-gas shielded arc welding with a filler metal that will allow the deposited metal to contain tantalum will thus form the deposited metal with improved resistance to cracking and intercrystalline corrosion and with high tensile strength.

The addition of less than 0.40% Ta will not be beneficial in improving the resistance to intercrystalline corrosion and more than 3.0% Ta will adversely affect the mechanical properties, particularly elongation, of the deposited metal. For these reasons the Ta addition should be limited within the range of 0.40 to 3.0%.

It will be also seen that Nb contents of less than 0.30% have no effect on welding crack.

in brief, according to the present invention, a welding material is prepared in which tantalum is contained in such an amount that from 0.4 to 3.0% of the element is left in the deposited metal formed in the welding of austenitic stainless steels, and the material is added heforehand to at least either filler metal (including the core wire for shielded arc welding) or flux, and then shielded arc welding, inert-gas shielded arc welding, submerged arc welding, or the like is carried out to produce a deposited metal having excellent crack resistance, corrosion resistance, and high-temperature strength. The invention thus has great industrial advantages.

What is claimed is:

1. In a welding material for austenitic stainless steel with a chemical composition effective to produce a deposited metal having the following composition:

not more than 0.15% C, 15.0 30.0% Cr, 8.0

40.0% Ni, not more than 2.5% Mn,

not more than 1.5% Si,

not more than 3.0% Mo,

not more than 0.045% P,

not more than 0.030% S,

not more than 0.030% Nb, and

not more than 5% ferrite, the improvement which comprises said material having an amount of TA effective to produce a Ta content in the deposited metal of 0.40 3.0%.

2. In a coated electrode for shielded arc welding of austenitic stainless steel with a chemical composition effective to produce a deposited metal having the following composition:

not more than .15% C 15.0 30.0% Cr 8.0 20.0%

not more than 2.5% Mn not more than 0.9% Si not more than 0.04% P not more than 0.03% S not more than 0.30% Nb and not more than 5% ferrite, the improvement which comprises either the core wire or coating of the electrode having an amount of Ta effective to produce a Ta content in the deposited metal of 0.40-3.0%.

3. A core wire for welding austenitic stainless steels consisting essentially of not more than 0.15% C, 15.0 30.0% Cr, 8.0 20.0% Ni, not more than 2.5% Mn, not more than 0.9% Si, not more than 0.04% P, not more than 0.03% S, not more than 0.30% Nb, and 0.44 5.0% Ta, the balance being iron and incidental impurities. 

1. IN A WEIGHT MATERIAL FOR AUSTENITIC STAINLESS STEEL WITH A CHEMICAL COMPOSITION EFFECTIVE TO PRODUCE A DEPOSITED METAL HAVING THE FOLLOWING COMPOSITION NOT MORE THAN 0.15% C, 15.0 - 30.0% CR, 8.0 - 40.0% NI, NOT MORE THAN 2.5% MN, NOT MORE THAN 1.5% SI, NOT MORE THAN 3.0% MO, NOT MORE THAN 0.045% P, NOT MORE THAN 0.030% S, NOT MORE THAN 0.030% NB, AND NOT MORE THAN 5% FERRITE, THE IMPROVEMENT WHICH COMPRISES SAID MATERIAL HAVING AN AMOUNT OF TA EFFECTIVE TO PRODUCE A TA CONTENT IN THE DEPOSITED METAL OF 0.40%.
 2. In a coated electrode for shielded arc welding of austenitic stainless steel with a chemical composition effective to produce a deposited metal having the following composition: not more than .15% C 15.0 - 30.0% Cr 8.0 - 20.0% Ni not more than 2.5% Mn not more than 0.9% Si not more than 0.04% P not more than 0.03% S not more than 0.30% Nb and not more than 5% ferrite, the improvement which comprises either the core wire or coating of the electrode having an amount of Ta effective to produce a Ta content in the deposited metal of 0.40-3.0%.
 3. A core wire for welding austenitic stainless steels consisting essentially of not more than 0.15% C, 15.0 - 30.0% Cr, 8.0 - 20.0% Ni, not more than 2.5% Mn, not more than 0.9% Si, not more than 0.04% P, not more than 0.03% S, not more than 0.30% Nb, and 0.44 - 5.0% Ta, the balance being iron and incidental impurities. 